1. Field of the Invention
The field of art to which this invention pertains includes cargo handling devices and more specifically the means of actuation and control of the twistlocks of a container lifting spreader.
2. Description of Related Art
The handling of large freight containers is a common activity in the freight transportation industry. These containers are handled by specialized load handling attachments, known as spreaders, which are mounted on various types of machines; including cranes, straddle carriers and large lift trucks. The spreader is a beam, often capable of telescoping to handle containers of different lengths, which is releasably coupled to the container by rotatable twistlocks mounted at the ends of the beam in position for insertion into mating connectors formed by oval apertures in castings at the corners of the containers. After insertion, the twistlocks are rotated, commonly by hydraulic cylinder assemblies, to a locked position to couple the container to the spreader for lifting.
Since the container and the handling machine may be supported in different planes, proper insertion of the twistlocks in all corner castings is not automatially assured. Likewise, the operator may have difficulty determining whether a corner of the container remains supported by one, or more, twistlocks when depositing the container. Many spreaders include an interlock to prevent the rotation of the twistlocks to the locked or unlocked position unless the twistlocks have been inserted fully in the corner castings and there is no weight bearing on a twistlock.
In the prior art, both mechanical and electrical interlocks have been utilized. An electrical interlock is disclosed in U.S. Pat. No. 3,885,676. Electrical switches detect contact of the spreader with the container at the twistlock locations. When the switches all indicate contact, a circuit is closed through appropriate relays to a solenoid operated hydraulic valve. The operator, by actuation of a switch, causes the shifting of the hydraulic valve to direct fluid to hydraulic cylinders to rotate the twistlocks. Electrical interlock systems are complex and expensive; requiring components to prevent the system from being affected by extraneous electromagnetic fields or power interruption, large solenoid operated hydraulic valves and some means of providing electrical power at the ends of the spreader which may telescope to handle different lengths of containers. The effects of moisture and salt, present in the air at marine terminals, on electrical components can result in costly maintenance requirements for these systems.
Mechanical interlocks; such as disclosed in U.S. Pat. No. 4,402,543; are less complex and costly than the electrical interlock. In this interlock a spring biased pin mechanically interferes with the rotation of the twistlock unless the pin is depressed by the weight of the spreader when it contacts the container at the twistlock locations. The pin must resist the full force of the twistlock rotating mechanism and force limiting devices may be required to avoid overriding the interlock or damaging the rotating mechanism. Further, if the operator should attempt to operate the twistlock before the pin is completely depressed, the pin may bind and fail to operate properly.